DIY Bookshelf speaker enclosure - Best dimensions

Speakers behave differently based on its type, dimensions, the box in which it is placed etc. However, there are some magic dimensions that works good for a specific speaker size (Please find it in the table below). These dimensions work well for two-way bookshelf speakers.

Too much big the enclosure, the sound becomes boomy. Too little the enclosure and it will resonate in the vocal frequencies. Only an appropriate enclosure produces a good punchy bass. Exceptions are 2 and 3 inch drivers. However, you can expect some virtual bass from 2 or 3 inch drivers too, with good stereo imaging. 

For 2 and 3 inch drivers, a port at 120 Hz may work good, depending on the speaker's capability.

Speaker size   (Inches)	Enclosure volume  (Cubic feet)
2	0.07
3	0.11
4	0.145
5.25	0.26
6.5	0.32
8	0.395

See also : Magic speaker enclosure dimensions for great sounding loudspeakers